Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222016

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
316 PM CDT Mon Apr 22 2019

315 PM CDT

Through Tuesday...

Main concern is the isolated risk for strong to severe storms
through early evening across portions of northwest, north central
and perhaps far northern Illinois through early evening. SPC made
no changes to the Day 1 level 1/Marginal Risk, which appears
reasonable. Temperatures in threat area have warmed into the
upper 70s to near/around 80, though amidst dew points only in the
lower to mid 50s. 18z DVN RAOB shows the advertised environment
with steep low and mid-level lapse rates and effective deep layer
shear of 40+ knots. However, the dew points only being in the 50s
continue to enable the base of the EML around 850 mb to keep cap
to surface based convection in place. DVN office has issued a
few warnings for severe hail for the cluster near the Quad Cities
around and shortly after 3pm CDT. General environment will remain
through shortly after sunset and expectation is for increasing
ascent from approaching short-wave trough and mid-level speed max
around 50 kt to erode the capping in place.

Will continue to monitor lead clusters near QCA as they track
northeast into the late afternoon for strong/severe risk.
Anticipating that further increase in coverage of convection
(scattered to numerous thunderstorms) will be during the early
evening hours with the approach of surface cold front trailing
from low pressure passing just to our north. Prior to nocturnal
stabilization of the boundary layer, steep low and mid-level
lapse rate and 35-45 kt of effective shear combo will support an
isolated strong to severe thunderstorm capable of hail and/or
damaging wind gusts in the marginal risk area. Threat should
minimize with eastern extent beyond sunset and it remains
uncertain the coverage of thunderstorms as area of convection
moves into immediate Chicago area during the mid to late evening.
Forcing lifting northeast of the area and increasing CIN should
greatly reduce coverage of any showers with cold frontal passage
to isolated after midnight into early Tuesday. Lowest levels will
remain sufficiently mixed for southwest gusts to 30-35 mph through
late evening ahead of the cold front. After a mild evening
outside of convection, temps will cool to primarily 50s by early
Tuesday behind the cold front.

On Tuesday, winds will turn north-northwesterly area wide, and
then northeasterly lakeside due to lake influence. Visible
satellite and observational data shows a fairly large area of
stratus behind the system cold front that will move in late
tonight into Tuesday morning. High pressure and dry advection from
the north will then erode the low clouds from north to south
Tuesday afternoon. Cooler air mass and clouds through portion of
peak heating will limit high temps to the lower to perhaps mid
60s inland on Tuesday, with only low-mid 50s near Lake Michigan.
It`s possible a few spots on immediate shore stay in upper 40s.




Tuesday Night through Monday...

230 PM...High pressure will build across the Great Lakes region
Tuesday night and slowly shift east on Wednesday. The frontal
boundary that pushes through the area Tuesday will stall south of
the cwa Tuesday night into Wednesday but the ecmwf rides a weak
low along this front which brings showers back into the area by
Wednesday morning. Will trend toward the drier nam/gfs and keep
the forecast dry but this time period will need to be monitored
for possible addition of pops. A cold front will then move across
the area Thursday with another chance of showers and
thunderstorms. Temps may rebound into the 70s on Thursday ahead of
the cold front. Models differ on the track of the next system for
Saturday but this system should bring another chance of precip to
the area Saturday/Saturday evening. cms


For the 18Z TAFs...

1246 PM...Forecast concerns include...

Strong/gusty south/southwest winds into this evening.
Wind shift to northwest Tuesday morning.
Wind shift to northeast Tuesday afternoon.
Chance of showers this afternoon.
Chance of showers...possibly a few thunderstorms this evening.
Mvfr cigs overnight through early Tuesday afternoon.

Southerly winds are expected to turn a bit more to the south/
southwest this afternoon with speeds/gusts increasing. Forecast
soundings show fairly strong winds in the low levels and if these
were to mix to the surface...gusts into the lower/mid 30kt range
would be possible but confidence is low so will continue with
upper 20kt gusts which will continue into this evening as the low
levels remain mixed. Winds will turn more southwest later this
evening then shift northwest before daybreak Tuesday as a cold
front moves across the area. Winds will turn more northerly by
mid/late Tuesday morning. A lake breeze/wind shift is possible
Tuesday afternoon as high pressure approaches the area.

Earlier shower/thunderstorm activity has weakened/dissipated as
it moves east of the current instability. Much of the short term
guidance continues to show a small chance for showers across the
Chicago area this afternoon. Confidence is fairly low and these
should be low impact...perhaps just sprinkles...but opted to carry
tempo vfr showers given the consensus of the models. Scattered
thunderstorms are expected to form over northeast IA/far northwest
IL late this afternoon and then spread across northern IL this a weakening phase. Best chance for thunder will be at
rfd and while thunder can/t be ruled out further east...coverage
looks isolated so maintained just shower mention for now.

Cigs will lower to mvfr behind the cold front overnight with mvfr
cigs continuing into late Tuesday morning/early Tuesday afternoon.
Some brief ifr cigs are possible but prevailing cigs look to
remain low mvfr. cms


LM...Small Craft Advisory...nearshore waters
     until 3 AM Tuesday.




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